The cellular mechanism underlying the directional selectivity of retinal ganglion cells in many species is as yet poorly understood. A hypothesis postulates that this directionality is due to a spatially-asymmetric inhibitory input onto the ganglion cells. This asymmetric input would prevent responses to motions in the null-direction, but would not prevent preferred-direction responses. An alternative hypothesis assumes that preferred-null discrimination occurs in the dendrites of amacrine cells, which then relay this information to ganglion cells. These dendrites would provide a spatially asymmetric excitatory input onto the ganglion cell. Nonlinear processes in the dendrites would produce null-direction inhibition and facilitation of the responses to preferred-direction motions. The neurotransmitter GABA is believed to mediate the inhibition in both hypotheses. This grant proposal describes three experiments that will scrutinize these two hypotheses. The experiments will use ON-OFF directionally selective ganglion cells of turtles and rabbits. 1) The first experiment will test whether GABAergic inhibition via GABA-A receptors is necessary for directional selectivity. 2) The second experiment will test whether direct ganglion-cell inhibition is necessary for directional selectivity. 3) The third experiment will determine whether the excitatory receptive field reflects the spatial asymmetry necessary for directional selectivity. This project should contribute to the understanding of how biophysical mechanisms underlie directional selectivity. Furthermore, the study of retinal directional selectivity will have an impact on the understanding of eye-movement control.